In CMOS circuit in a semiconductor device, there are two states in a voltage level of a signal of a power supply voltage level as high level and a ground voltage level as low level, and the signal is transmitted at the voltage level. A change of the signal is performed by changing the voltage level of the signal from the ground voltage level to the power supply voltage level or from the power supply voltage level to the ground voltage level. However, there is a possibility that so-called an overshoot and an undershoot occur in which the signal reaches the voltage level exceeding an object power supply voltage level and ground voltage level due to a parasitic capacitance of a transmission circuit. Disorder of signal waveforms resulting from the overshoot and the undershoot causes a noise to deteriorate a transmission quality. Besides, the overshoot and the undershoot become large if high-speed is emphasized by enlarging a drive capability and so on to change the signal in high speed.
For example, a circuitry as illustrated in FIG. 8A is used to suppress the overshoot and the undershoot. FIG. 8A is a view illustrating an example of an output circuit. In FIG. 8A, a driver (output buffer) 81 is an inverter made up of, for example, CMOS transistors. The driver 81 includes a P-channel transistor 82 and an N-channel transistor 83 connected between wirings to which a power supply voltage Vdd and a ground voltage Gnd are respectively supplied. An input signal input to the output circuit is output from an output terminal out as an output signal via the driver 81.
In the output circuit illustrated in FIG. 8A, terminating resistors 84, 85 are connected to a signal line of the output signal. The terminating resistors 84, 85 are provided, and thereby, the effect caused by the parasitic capacitance of the transmission circuit is suppressed, excess of the voltage level of the signal over the power supply voltage level and the ground voltage level is suppressed, and the overshoot and the undershoot are reduced. FIG. 8B is a view illustrating waveforms of the output signal when the terminating resistors are connected as illustrated in FIG. 8A. In FIG. 8B, a signal waveform 86 illustrated by a solid line represents the output waveform when the terminating resistors are connected, and a signal waveform 87 illustrated by a dotted line represents an output waveform when the terminating resistors are not connected.
A method is proposed in which the overshoot and the undershoot are reduced by terminating operations of one driver from among two drivers outputting the output signals just before the voltage level of the signal reaches the objective power supply voltage level or ground voltage level (for example, refer to Patent Document 1). A method is proposed in which one driver is operated until the voltage level of the signal reaches a certain voltage level and two drivers are operated after the signal reaches the certain voltage level when the output signal changes (for example, refer to Patent Documents 2, 3).    [Patent Document 1] Japanese Laid-open Patent Publication No. 03-242020    [Patent Document 2] Japanese Laid-open Patent Publication No 05-243958    [Patent Document 3] Japanese Laid-open Patent Publication No. 03-89624
As stated above, the terminating resistors are connected to the signal line of the output signal, then the overshoot and the undershoot are reduced as illustrated in FIG. 8B. However, an amplitude level of the signal becomes small because the voltage level at a steady time does not reach the power supply voltage level and the ground voltage level caused by an effect of the terminating resistors. Besides, the voltage level in reverse to a signal change is applied for the signal line of the output signal from a power supply or a ground voltage (ground) connected via the terminating resistors, and thereby, a time required from a start to an end of the change of the output signal increases, and the signal change delays. Besides, the signal line is connected to the power supply and the ground voltage (ground) via the terminating resistors, and thereby, a leak current occurs constantly, and power consumption increases.
In the art described in Patent Document 1, the overshoot and the undershoot are reduced by terminating the operations of one driver. However, the overshoot and the undershoot occur due to the parasitic capacitance of the transmission circuit because the other driver continues to operate. Note that it is conceivable to reduce a voltage change amount per certain time as a method to suppress the overshoot and the undershoot, but the time required for the change of the output signal increases and the high-speed operation becomes impossible.